Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds (Phaethornithinae): a role for ecological causation

Abstract

Unambiguous examples of ecological causation of sexual dimorphism are rare, and the best evidence involves sexual differences in trophic morphology. We show that moderate female-biased sexual dimorphism in bill curvature is the ancestral condition in hermit hummingbirds (Phaethornithinae), and that it is greatly amplified in species such as Glaucis hirsutus and Phaethornis guy, where bills of females are 60 per cent more curved than bills of males. In contrast, bill curvature dimorphism is lost or reduced in a lineage of short-billed hermit species and in specialist Eutoxeres sicklebill hermits. In the hermits, males tend to be larger than females in the majority of species, although size dimorphism is typically small. Consistent with earlier studies of hummingbird feeding performance, both raw regressions of traits and phylogenetic independent contrasts supported the prediction that dimorphism in bill curvature of hermits is associated with longer bills. Some evidence indicates that differences between sexes of hermit hummingbirds are associated with differences in the use of food plants. We suggest that some hermit hummingbirds provide model organisms for studies of ecological causation of sexual dimorphism because their sexual dimorphism in bill curvature provides a diagnostic clue for the food plants that need to be monitored for studies of sexual differences in resource use.

Figure 1.

Sexual dimorphism in (a) bill curvature, (b) wing length, (c) bill chord and (d) bill arc for 30 hummingbird species included in the present study (21 Phaethornithinae and nine Trochilinae species). Positive values indicate female bias in dimorphism, whereas negative values indicate male bias. Phaethornithinae are indicated by open bars and Trochilinae by closed bars.

Figure 2.

Sexual dimorphism in bill curvature in Phaethornithinae. Values in boxes are bill curvature dimorphisms calculated for extant taxa or reconstructed for ancestral species (§2). Black shading indicates bill curvature dimorphism greater than 30%, whereas white boxes represent moderate (approx. 25%) or no bill curvature dimorphism. Note that the non-hermit Eriocnemis luciani is the only taxon with male-biased bill curvature dimorphism (grey shading). Arrows represent transitions (greater than 5%) in bill curvature dimorphism for hermit hummingbirds, which are either towards increasing female bias (arrows up) or decreasing female bias (arrows down). Photographs of species depict female above and male below. (Phylogeny adapted from McGuire et al. (2007, 2008).)

Figure 3.

Sexual dimorphism in bill curvature becomes increasingly more female-biased with increasing bill length, as measured by both (a,c) average bill chord and (b,d) average bill arc for (a,b) all species and (c,d) hermit hummingbirds only. Regressions of raw variables were performed in JMP, v. 5.0.1a (SAS Institute, 1989–2002) and phylogenetically independent contrasts (PIC) were carried out in Mesquite, v. 2.6 (Maddison & Maddison 2009). (a) y = 0.0309x − 0.5794; r² = 0.5705; F_1,28 = 37.19, p < 0.0001; PIC: r > 0.482, p ≤ 0.007. (b) y = 0.0262x − 0.4113; r² = 0.4768; F_1,28 = 25.51, p < 0.0001; PIC: r > 0.453, p ≤ 0.015. (c) y = 0.0294x − 0.5147; r² = 0.4763; F_1,19 = 17.3, p = 0.0005; PIC: r = 0.565, p = 0.008. (d) y = 0.0234x − 0.3121; r² = 0.2621, F_1,19 = 6.75, p = 0.0177; PIC: r = 0.472, p = 0.03.

Figure 4.

Bill and flower correspondence in two species of hermit hummingbirds. (a) Phaethornis guy. A straight-billed male bird pictured with H. beckneri (above) and a female curve-billed bird with a flower of curved H. tortuosa (below). (b) Glaucis hirsutus. A male straight-billed bird with H. hirsuta (above) and a female curve-billed bird with a curved flower from H. bihai (below). Photographs of H. beckneri and H. tortuosa from Taylor (2005); other photographs by E. Temeles.